The development of inorganic films or membranes which are selectively permeable to specific gases and are able to withstand the adverse environments encountered in most industrial processes is becoming increasingly important. Such membranes must be stable at high temperatures and resistant to chemical attack to be suitable for use in a combined process involving a catalytic reaction and product separation. Through the use of such selective permeation membranes, the yield of catalytic processes which are currently restricted by thermodynamic equilibrium can be significantly improved.
In U.S. Pat. No. 4,230,463, entitled "Multi-Component Membranes For Gas Separation", there is an overview of the development of membranes for use as separators. It starts with the development of membranes for liquid separation and proceeds to the development of membranes for gas separation, concluding with new multi-component membranes for gas separation. The membranes are all made from organic polymers and are used at temperatures below about 150.degree. C.
Polymeric membranes are unsuitable for high temperature separation as would be required in applications to catalytic processes. Inorganic membranes, on the other hand, can withstand high temperatures as well as oxidizing atmospheres. Both porous and non-porous inorganic membranes have been proposed for gas separations. Microporous membranes (pore size 40 to 50 .ANG.) have been investigated for hydrogen separation with or without simultaneous chemical reaction (see, e.g., Fukuda et al, Ind. Eng. Chem. Fundam., 17, 1978; Kameyama et al, ibid, 20, 97, 1981a; Kameyama et al, Hydrogen Energy Progress, 2 , 569, 1981b; Shindo et al, J. Chem. Eng. Japan, 16, 120, 1983; Shindo et al, ibid, 17, 650, 1984; Shindo et al, ibid, 18, 485, 1985; Haraya et al, ibid, 19, 186, 1986a; Haraya et al, ibid, 19, 461, 1986b).
In such microporous membranes, permeation is governed by Knudsen diffusion and the ideal permeability ratio that can be achieved is inversely proportional to the ratio of molecular weights of gases. For example, the ideal permeability ratio for the H.sub.2 -N.sub.2 pair is 3.74. Such ratios are not adequate for application to equilibrium-limited catalytic reactions.
Non-porous membranes are capable of larger selectivity ratios by virtue of the highly specific mechanism of solid state diffusion. Certain metals exhibit selective permeability to hydrogen.
U.K. Patent Application No. 2,190,397, entitled "Production of Aromatics From Alkanes", teaches the catalytic dehydrocyclodimerisation of C.sub.2 to C.sub.6 alkanes to a mixture of aromatic hydrocarbons and hydrogen. The reactor is fitted with a membrane which is capable of transferring at least a portion of hydrogen in the gaseous reaction products across the membrane and out of the reaction zone. The membrane can be made from the following metals or their alloys: Pd, Ti, Zr, Ni, Co, Fe, Pt, V, Nb, Ta and Ag. The preferred membrane is a palladium-silver alloy (76% Pd/24% Ag w/w).
An article by R. R. McCaffrey et al, entitled "Inorganic Membrane Technology" and published in Separation Science and Technology 22 (213), 873-887, 1987, reported that films of poly-bis (trifluoroethoxy) phosphazene are highly permeable to methanol, ethanol, isopropanol, and phenol. The films, however, melted at 238.degree. C.
The McCaffrey et al article also notes that the permeability of tungsten, molybdenum, copper, nickel, iron and alloys of these metals have been studied but in most cases only for the permeation and diffusion of hydrogen and its isotopes, as well as oxygen, nitrogen and in some cases carbon monoxide. It states that palladium and its silver alloys are widely used to purify hydrogen, and silver is quite permeable to oxygen.
The authors also reported deposition films of vanadium and aluminum on a VCR filter gasket assembly to determine adherence, microstructure and He leak-rate, but no results are given other than to note that the films flaked off after exposure to laboratory air over a period of one week.
It appears that the only separation that has been commercially carried out is that of hydrogen by palladium or palladium alloy membranes. The use of noble metals entails considerable cost and may be adversely affected by sulfur poisoning when the gas mixture contains hydrogen sulfide or other sulfur compounds.
Silica glass (main constituent SiO.sub.2) is known to be highly selective for permeation by hydrogen V. O. Altemose, 7th Symposium on the Art of Glassblowing, The American Glassblowers Society, Wilmington, Del., 1962 reported data showing that at 500.degree. to 1,000.degree. K., the H.sub.2 : O.sub.2 separation factor is higher than 10,000. The permeability itself, however, is very low, so that a practical separation device would have to consist of an extremely thin film (less than 1 .mu.m) of SiO.sub.2. Production of films of such thickness in a form which is suitable for gas separation has not been reported in the prior literature.